The present invention relates to a process photometer for monitoring a production process in which such photometer is used.
In-line monitoring of chemical processes makes it possible to reduce production cycle time, improve the safety of such processes and reduce waste. A number of methods for such monitoring with conventional analytical devices have been disclosed in the literature. Two general approaches have been taken in developing such monitoring processes. In one approach, the amount of energy absorbed by a sample or selected material is measured and compared to one or more pre-established standards. Examples of this approach may be found in U.S. Pat. Nos. 5,151,474; 5,170,056; and 5,763,883.
U.S. Pat. No. 5,151,474, for example, discloses a polyolefin polymerization process in which the rate of addition of the principal monomer is controlled on the basis of a vibrational spectroscopy analysis (e.g., Fourier Transform infrared spectroscopy) of that monomer's concentration in the reactor. In this method, the chemical analysis may be conducted either on-line or off-line. On-line analysis is conducted by directing a portion of the process stream directly to the process analyzer. Such diversion is not, however, desirable in many production environments.
U.S. Pat. No. 5,170,056, discloses an infrared probe for in situ sensing of infrared energy absorption in a sample. This probe which senses the amount of infrared energy absorbed in a sample is designed to make it possible to determine infrared spectroscopic signatures at a point remote from a standard spectrometer.
U.S. Pat. No. 5,763,883 discloses a method for determining or predicting a value of a selected property of a material being produced in which the absorption of the selected material is measured at more than one wavelength and the measured wavelengths are compared to standards.
Another approach taken in monitoring commercial chemical production processes is the photometric approach in which the radiation absorbed by a sample at predefined wavelengths and bandwidths is evaluated in relation to a pre-established standard. Examples of this photometric approach are found in U.S. Pat. Nos. 4,968,148 and 5,825,478.
U.S. Pat. No. 4,968,148 discloses a single source multi-site photometric system in which a number of isolated samples on a sample plate may be analyzed for kinetic and/or end-point densitometric properties in rapid sequence. This system is not, however, capable of monitoring a production process as that process is being conducted. The samples must be collected and analyzed at a remote location.
U.S. Pat. No. 5,825,478 discloses a multifunctional photometer in which electromagnetic radiation from a source which has been sent through a sample cell is analyzed substantially instantaneously by multiple detectors at multiple wavelengths from a broad band electromagnetic radiation source. The use of multiple detectors, however, increases the costs associated with the construction and maintenance of the photometer. In addition, the use of beam splitters to direct a portion of the electromagnetic radiation to each detector reduces the total signal that would be available if only one detector were used. In many cases, the energy available at the detector has been attenuated to marginal levels either by the constituent of interest, or scattering from suspended particles.
An accurate photometric device having a minimum number of components which is durable, inherently stable, and compact, may be housed in an explosion proof enclosure, that does not require instrument air or nitrogen purging for explosion proof rating or cooling, and which has been optimized for maximum optical throughput and signal to noise ratio, so that it can be permanently installed at a location close to the stream of material to be monitored would be advantageous.